Fungal biomass and fungal extract of the nonpathogenic fungus Neurospora crassa were successfully used as reducing agents for the biosynthesis of platinum nanoparticles (PtNPs). The experiment was carried out by exposing the fungal biomass or the fungal extract to a 0.001 M precursor solution of hexachloroplatinic(IV) acid ($H_2PtCl_6$). A change of color of the biomass from pale yellow to dark brown was the first indication of possible formation of PtNPs by the fungus. Subsequent analyses confirmed the intracellular biosynthesis of single PtNPs (4-35 nm in diameter) and spherical nanoaggregates (20-110 nm in diameter). Using the fungal extract, similar results were obtained, producing rounded nanoaggregates of Pt single crystals in the range of 17-76 nm.